1. Field of the Invention
This invention relates to a bridge balancing method and to a thermally controlled stage that is connected within one arm of a capillary bridge viscometer so that the temperature of the one arm can be controlled to a different temperature than the temperatures of the other three arms. This allows the bridge be automatically balanced to provide accurate measurement signals.
2. Background Art
An example of a capillary bridge viscometer which is adapted to accurately measure the specific viscosity of a solute in a suitable solvent is available by referring to U.S. Pat. No. 4,463,598 issued Aug. 7, 1984. Such a capillary bridge viscometer is useful in determining the molecular parameters of a polymer including molar mass and hydrodynamic radius. FIG. 1 of the drawings illustrates this well known capillary bridge viscometer.
The aforementioned patented differential viscometer includes a capillary bridge 50 that must be balanced to achieve accurate test results and a wide dynamic range. A solvent is usually supplied from a reservoir to the capillary bridge 50 by means of a low pulsation chromatography pump. Typically, the sensitivity of bridge 50 is limited by the pressure amplitude of the pump pulses. First and second capillaries 52 and 54 are connected in series between supply and discharge ports 60 and 62. Third and fourth capillaries 64 and 66 are connected in series between the supply and discharge ports 60 and 62. The series connected capillaries 52, 54, and 64, 66 are connected in parallel with one another between the supply and discharge ports 60 and 62 to form a fluid analog of the well known Wheatstone (i.e., resistance) bridge in the electrical art.
A delay volume 72 is located in the fluid arm of bridge 50 which contains the capillary 66. The delay volume 72 is constructed so as to have a negligible flow impedance, but a large internal volume. A differential pressure transducer 74 is connected in the capillary bridge 50 to measure the differential pressure across the bridge when different fluids are flowing through the capillaries thereof. Another differential transducer 75 is connected between the supply and discharge ports 60 and 62 to measure the pressure from the inlet to the outlet sides of the bridge. Typically, a zero reading of the pressure transducer 74 provides an indication that the bridge 50 is in balance.
In the traditional method for mechanically tuning the bridge 50, the length of one fluid arm of the bridge is changed by disassembling the bridge and precisely cutting off (or adding) a length of tubing. This is generally tedious and time consuming. Moreover, some fluid samples such as proteins, and the like, are known to stick to the fluid tubing which causes the original tuning to slowly drift with time. In this case, the fluid tubing must be cleaned out and flushed by the operator or a periodic rebalancing will otherwise be required. In the alternative, the viscometer will have to be returned to its manufacturer to be serviced. In either case, the viscometer will be rendered temporarily out of use with the consequence that fluid sample testing will be inefficiently delayed. What would be desirable is an improved balancing technique that is equivalent in effect to the mechanical balancing, but can be accomplished automatically and more accurately, as required, and without disassembly of the system.